Q.For the reaction A → B, the rate of reaction becomes three times when the concentration of A is increased by nine times. What is the order of reaction ?
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Order and Molecularity: The Two Faces of a Reaction
Imagine you are watching a chemical reaction happen. Two molecules of A collide, rearrange, and become B. That collision — the actual event where bonds break and form — is an elementary step. The number of molecules that must come together in that single step is its molecularity.
Now imagine a different reaction. You mix A and B, but the product appears at a rate that depends on the square of A and not at all on B. That dependence — how the speed of the overall reaction changes when you change concentrations — is its order.
These two ideas answer different questions. Molecularity asks: How many particles actually meet in one step? Order asks: What is the mathematical relationship between concentration and rate for the overall reaction?
Molecularity — The Microscope View
Molecularity is a property of an elementary reaction only. It is the number of reactant particles (atoms, ions, molecules) that collide in that step.
- Unimolecular (1): A single molecule decomposes or rearranges. Example: N2O5→NO2+NO3
- Bimolecular (2): Two particles collide. Example: NO2+CO→NO+CO2
- Termolecular (3): Three particles collide simultaneously — very rare because three-way collisions are improbable. Example: 2NO+O2→2NO2
Molecularity is never zero, never fractional, and never greater than 3. You cannot have a molecularity of 1.5 or 4 — collisions of four particles at once are astronomically unlikely.
Order — The Macroscope View
Order is an experimental quantity. You run the reaction, measure the initial rate at different concentrations, and deduce the rate law:
Rate=k[A]m[B]n
The order with respect to A is m, the order with respect to B is n, and the overall order is m+n.
Order can be:
- Integer (0, 1, 2, 3)
- Fractional (e.g., 0.5 for a reaction like H2+Br2→2HBr)
- Negative (if increasing a reactant slows the reaction)
- Zero (if concentration does not affect rate)
Order is determined by experiment, not by the balanced chemical equation. A reaction written as 2A+B→C does not automatically have order 3. The actual rate law could be Rate=k[A]2 (order 2) or Rate=k[A] (order 1) — only the lab can tell.
The Critical Difference
| Property | Molecularity | Order |
|---|---|---|
| Applies to | Elementary steps only | Overall reaction (or any step) |
| Determined by | Stoichiometry of the step | Experiment |
| Can be fractional? | No | Yes |
| Can be zero? | No | Yes |
| Can be negative? | No | Yes |
| Maximum value | 3 (rarely) | Any number |
Why the Confusion?
Many students think: "If the balanced equation says 2A + B, then molecularity is 3 and order is 3." This is wrong for two reasons.
First, most reactions are multi-step. The balanced equation shows only the net change, not the actual collision events. A reaction like 2NO+O2→2NO2 appears termolecular, but it actually happens in two bimolecular steps:
NO+NO→N2O2(fast)
N2O2+O2→2NO2(slow)
The molecularity of each step is 2 (bimolecular). The overall order, determined by the slow step, is 2+1=3 — but that is a coincidence, not a rule. …
Using the rate law rate=k[A]n, if increasing [A] nine times makes the rate three times, then 3=9n=(32)n, so 2n=1. …
The order comes straight from the rate law: rate ×3 for concentration ×9 gives 9n=3, so n=21.
Concept. For A→B, the rate depends on [A] as rate=k[A]n, where n is the order with respect to A. This is a standard CBSE Class-12 Chemistry chemical-kinetics numerical.
Why. The order is the experimentally found exponent linking rate to concentration; we recover it by comparing two rates.
Steps. …
- GSEB Higher Secondary Certificate (HSC) Examination 2026Set ANNUAL1 markMCQQ.For a given reaction: KClO3 + 6FeSO4 + 3H2SO4 -> KCl + 3Fe2(SO4)3 + 3H2O The order of reaction is ____.(a) First(b) Second(c) Zero(d) Pseudo First Order Reaction
›Reveal solutionSolution
A balanced chemical equation only tells us the overall stoichiometry, not the rate law — the ORDER of a reaction must always be determined experimentally.
For the reaction:
KClO3 + 6FeSO4 + 3H2SO4 → KCl + 3Fe2(SO4)3 + 3H2O
If order were read naively off the stoichiometric coefficients (1 + 6 + 3), it would appear to be an implausibly high 10th-order reaction. In reality, this reaction proceeds through several elementary steps, and its rate is controlled by the slowest (rate-determining) step among them. Kinetic studies on this classic textbook example show the …
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.What is the unit of rate constant for Hydrogenation of Ethene.(a) S-1(b) mol-1 L S-1(c) mol L-1 S-1(d) mol L-2 S-1
›Reveal solutionSolution
Catalytic hydrogenation of ethene, rate = k[C2H4], is a standard textbook example of a first-order reaction, so its rate constant carries units of (time)-1.
For a reaction of overall order n, the rate constant's units are (concentration)^(1-n) x (time)^-1.
…
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.A reaction is First order in A and second order in B. When the concentration of both A and B are doubled. The rate will be increased _____ times.(a) 4(b) 8(c) 6(d) 2
›Reveal solutionSolution
Rate = k[A][B]^2; doubling both A and B multiplies the rate by 2 x 2^2 = 8.
Given: order in A = 1, order in B = 2, so rate law: Rate = k[A]^1[B]^2.
When [A] -> 2[A] and [B] -> 2[B]: …
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.For a given reaction :- KClO3 + 6FeSO4 + 3H2SO4 -> KCl + 3Fe2(SO4)3 + 3H2O. The order of reaction is _____.(a) First(b) Second(c) Zero(d) Pseudo First Order Reaction
›Reveal solutionSolution
Order of a reaction is an experimental quantity and is NOT decided by the stoichiometric coefficients of the balanced equation - this reaction is the classic textbook example of that principle.
The balanced equation KClO3 + 6FeSO4 + 3H2SO4 -> KCl + 3Fe2(SO4)3 + 3H2O involves 10 reactant molecules by stoichiometry, which might naively suggest a very high (tenth) order. But order can only be found by experiment, since it reflects the actual mechanism (rate-determining step), not the overall balanced equation.
…
- GSEB Higher Secondary Certificate (HSC) Examination 2024Set ANNUAL1 markMCQQ.If value of rate constant K = 2.3 x 10^-5 L mol-1 S-1, then identify the reaction order:(a) Second order(b) Third order(c) First order(d) Zero order
›Reveal solutionSolution
The units of a rate constant reveal the overall order of the reaction, since rate = k[conc]^n and rate always has units of mol L-1 s-1.
For a reaction of order n: k has units of (mol L-1)^(1-n) s-1.
Zero order: mol L-1 s-1
First order: s-1
Second order: L mol-1 s-1 (i.e., mol-1 L s-1)
Third order: L2 mol-2 s-1
…
- GSEB Higher Secondary Certificate (HSC) Examination 2023Set ANNUAL1 markMCQQ.What will be the unit of rate constant for following reaction? C2H4(g) + H2(g) -> C2H6(g)(a) mol^-2 L2 S^-1(b) S^-1(c) mol^-1 L S^-1(d) mol L^-1 S^-1
›Reveal solutionSolution
For an overall second-order reaction, k has units mol^-1 L s^-1.
Rate = k [C2H4][H2] is second order overall (order = 2). The unit of a rate constant is:
unit of k = (mol L^-1)^(1-n) s^-1, where n = overall order. …
- GSEB Higher Secondary Certificate (HSC) Examination 2022Set ANNUAL1 markMCQQ.What is the unit of the rate constant for a second-order reaction?(a) mol L-1 s-1(b) s-1(c) mol-1 L s-1(d) mol-2 L+2 s-1
›Reveal solutionSolution
Unit of k = (unit of rate) / (unit of concentration)^order.
Rate = k[A]^2 for a second-order reaction. …
- GSEB Higher Secondary Certificate (HSC) Examination 2019Set ANNUAL1 markMCQQ.Which of the following relation is correct for elementary bimolecular reaction?(a) Order of reaction <= molecularity(b) Order of reaction > molecularity(c) Order of reaction = molecularity(d) Order of reaction < molecularity
›Reveal solutionSolution
For any elementary (single-step) reaction, the rate law can be written directly from the balanced equation, so order and molecularity coincide.
Molecularity is the number of reacting species (atoms/ions/molecules) that collide simultaneously in a single elementary step; it is always a whole number and is a theoretical/mechanistic concept. Order of reaction is the experimentally determined sum of the powers of concentration terms in the rate law. For a COMPLEX (multi-step) reaction these can differ, since the rate law follows the slow (rate-determining) step, not the overall stoichiometry. But for an ELEMENTARY reaction -- such as a simp …
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